Leaks of hazardous substances from storage tanks pose a substantial threat to the environment in various areas throughout the world. It is estimated that there are approximately 1.4 million underground storage tanks in the United States alone that contain commercial quantities of petroleum or other hazardous substances. An estimated 100,000 of these tanks are presently leaking and another 350,000 are expected to leak within the next five years. New York estimates that 19% of its 83,000 active underground gasoline tanks are now leaking. Maine estimates that 25% of its 1,600 retail gasoline underground storage tanks leak approximately eleven million gallons per year.
The corrective costs associated with such leaking tanks are staggering. The gasoline leakage from one tank may require an expenditure of up to ten million dollars for a proper clean up. For example, leakage from an underground gasoline tank on Cape Cod has infiltrated the underground aquifier in the area and three million dollars already has been spent in unfinished corrective actions.
The existing methods of leak detection exhibit various drawbacks, including extremely long testing periods, inability to differentiate between piping and tank leaks, failure to account for vapor pockets, and failure to compensate for water table influences. Additionally, many of the existing tests are not sensitive enough and do not yield reliable results. For example, The National Fire Protection Association (NFPA) codes require tank replacement or corrective action for leak rates greater than 0.05 gallons per hour. Thus, an adequate leak testing method must be sensitive enough to measure leaks of 0.05 gallons per hour or less. Existing leak detection devices have not been able to meet this requirement with a high degree of reliability. Whereas most states have adopted the NFPA regulations, the Environmental Protection Agency may require that all commercial underground storage tanks have a leak rate less than 0.1 gal/hr. with a 99.9% reliability factor.
The current invention involves an electromechanical leak detection device which detects minute volumetric changes in fluid levels in storage tanks or their associated piping systems. The invention detects and measures leaks by measuring buoyancy force changes on a mechanical force transducer inserted into the liquid in a storage tank. The buoyancy force changes result from liquid level changes inside the tank which in turn cause moment forces acting on the mechanical force transducer. These moment forces cause strain changes in the mechanical force transducer. These strain changes are measured by a strain gage attached to the mechanical force transducer.
The invention is very sensitive being capable of detecting volume changes as little as 0.005 gallons per hour. The invention requires short testing time and is inexpensive both as an instrument and in practicing the method contemplated by the invention. The device of the invention is easily tared and calibrated and can distinguish between fill pipe leaks and tank leaks. A preferred embodiment of the invention is an automated device requiring minimal operator input.
Applicant's device for detecting and measuring leaks into or out of fluid storage tanks includes a support member, a mechanical force transducer and an electronic strain detection device. The free end of the mechanical force transducer terminates in a fluid force sensor. The other end of the mechanical force transducer is rigidly attached to the support member. The mechanical force transducer defines first and second segments substantially at right angles to one another. The second segment is capable of micro elongation and contraction when a force is applied axially to the first segment. Preferably the mechanical force transducer is constructed entirely of aluminum. The first segment is a rod rigidly attached to and extending at about a 90.degree. angle from the second segment, a cantilever beam capable of micro strain elongation and contraction when a force is applied axially to the rod. Preferably the strain detection device is a strain gage attached to the upwardly facing surface of the cantilevered beam close to its point of rigid attachment to the support member. The strain gage is attached to a strain gage meter.
In operation, the terminal end of the rigid force transducer is introduced into the fluid in a storage tank. The support member is maintained in fixed position. This maintains the force transducer with the terminal end submersed essentially in fixed position. The change in the force exerted by the fluid on the submerged end due to changes in fluid level results in micro elongations or contractions in regions of the mechanical force transducer. These elongations or contractions may be related to volumetric changes in the storage tank resulting from leaks out of or into the tank by calibrating units of micro strain elongation relative to known changes in the volume of fluid in the tank.